Stamp forming optimization for formability and crystallinity

Abstract

The stamp forming process is well suited for high volume production of thermoplastic composite parts. The process can be characterized as highly non-isothermal as it involves local quench-cooling of a molten thermoplastic composite blank where it makes contact with colder tooling. The formability of the thermoplastic composite depends on the viscoelastic material behavior of the matrix material, which is sensitive to temperature and degree of crystallinity. An experimental study was performed to determine the effect of temperature and crystallinity on the storage modulus during cooling for a woven glass fiber polyamide-6 composite material. An increase of two decades in modulus was observed during crystallization. As this will significantly impede the blank formability, the onset of crystallization effectively governs the time available for forming. Besides the experimental work, a numerical model is developed to study the temperature and crystallinity throughout the stamp forming process. A process window can be determined by feeding the model with the experimentally obtained data on crystallization.

Advances in composite forming through 25 years of ESAFORM

“…Several other isothermal approaches are available [230,231]. However, processing experiments reveal a distinct temperature-dependency [232,233]. Thus, coupled thermomechanical approaches with temperaturedependent shear and bending have been developed to capture the influence of the transient temperature on the deformation behaviour [229,234].…”

“…Thus, Guzman-Maldonado et al [225] developed a thermomechanical approach based on alternating thermal and mechanical simulations. At lower temperatures, the onset of crystallisation may induce a significant increase in mechanical stiffness [233]. Therefore, Dörr et al [190,235] developed a coupled thermomechanical approach, considering the phase transition from the molten to the solid material state through crystallisation for semi-crystalline thermoplastics, see Fig.…”

On the Relevance of Thermomechanics and Crystallization Kinetics for Finite Element Thermoforming Simulation

“…Both studies are restricted to temperatures well above the onset of crystallization of the investigated semi-crystalline thermoplastics. The onset of crystallization, however, induces a significant increase of mechanical properties [17,18]. Thus, it is to be expected that crystallization distinctively influences forming behavior.…”